The magnetic tunnel junction (MTJ) basically comprises two electrodes, which are layers of ferromagnetic material, separated by a tunnel barrier layer, which is a thin layer of insulating material. The tunnel barrier layer must be sufficiently thin so that there is a probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. The tunneling probability is spin dependent, however, depending on the availability of tunneling states with different electron spin orientations. Thus, the overall tunneling current will depend on the number of spin-up vs. spin-down electrons, which in turn depends on the orientation of the electron spin relative to the magnetization direction of the ferromagnetic layers. Thus, if these magnetization directions are varied for a given applied voltage, the tunneling current will also vary as a function of the relative directions. As a result of the behavior of an MTJ, sensing the change of tunneling current for a fixed potential can enable a determination of the relative magnetization directions of the two ferromagnetic layers that comprise it. Equivalently, the resistance of the MTJ can be measured, since different relative. magnetization directions will produce different resistances.
The use of an MTJ as an information storage device requires that the magnetization of at least one of its ferromagnetic layers can be varied relative to the other and also that changes in the relative directions can be sensed by means of variations in the tunneling current or, equivalently, the junction resistance. In its simplest form as a two state memory storage device, the MTJ need only be capable of having its magnetizations put into parallel (low resistance) or antiparallel (high resistance) configurations (writing data) and that these two configurations can be sensed by tunneling current variations or resistance variations (reading data). In practice, the free ferromagnetic layer can be modeled as having a magnetization which is free to rotate but which energetically prefers to align in either direction along its easy axis (the direction of magnetic crystalline anisotropy). The magnetization of the fixed layer may be thought of as being permanently aligned in its easy axis direction. When the free layer is anti-aligned with the fixed layer, the junction will have its maximum resistance, when the free layer is aligned with the fixed layer, the minimum resistance is present.
In typical MRAM circuitry, the MTJ devices are located at the intersection of current carrying lines called word lines and bit lines. When both lines are simultaneously activated, information gets written on the device, i.e. the magnetization direction of its free layer is changed. When only one line is activated, the resistance of the device can be sensed, so the device is effectively read.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 5,650,958 (Gallagher et al) discloses a barrier layer between free and pinned layers while U.S. Pat. No. 6,166,948 (Parkin et al) shows a multi-layer free layer structure and U.S. Pat. No. 6,665,155 (Gill) describes an amorphous cobalt niobium or cobalt hafnium layer between two free layers.
In U.S. Pat. No. 5,966,012 (Parkin), U.S. Pat. No. 6,839,206 (Saito et al), and U.S. Pat. No. 6,756,237 (Xiao et al) use of an amorphous barrier layer is disclosed and, in U.S. Pat. No. 6,831,312, Slaughter et al. teach at least one amorphous layer for smoothness so that the free layer and the ferromagnetic layers, for example, may comprise amorphous alloys of CoFeB.
U.S. Pat. No. 6,818,458 (Gill) shows an amorphous alloy of CoFeX as a ferromagnetic layer for smooth growth of a thin barrier layer while U.S. Pat. No. 6,452,762 (Hayashi et al) teaches that the fixed layer and the free layer may be of amorphous material. In U.S. Pat. No. 6,703,654 (a Headway patent by Horng et al) a smooth bottom electrode is disclosed. Finally, U.S. Patent Publication 2004/0229430 (Findeis et al) describes a free layer comprising multiple layers including amorphous CoFeB alloys.
Because of their greater dR/R and very low Hc, MTJ devices with amorphous CoFeB free layers are preferred for ultra high density and low power MRAM applications. In practice, however, the extremely high positive magnetostriction of CoFeB is a source of problems such as a widely varying switching field distribution among MRAM arrays. The present invention discloses how these problems can be overcome through suitable control of the composition and structure of free layer films.